1. Field of the Invention
This invention relates to a focusing technique that may be used with any type of unpatterned specular substrate and, more particularly, relates to a focusing technique for a microlithographic stepper that employs a Half-Field Dyson projection optical system to achieve focus of an image of a reticle pattern on a completely unpatterned surface of a wafer.
2. Description of the Prior Art
In the fabrication of integrated circuits, a microlithographic stepper is used to image each of a stack of successive circuit-patterns on the surface of a silicon or GaAs wafer supported on a movable wafer stage of the stepper. The stepper, in response to control data applied thereto, is capable of precisely moving its wafer-supporting stage so as to bring each of the successively applied circuit-patterns of the stack into substantially perfect alignment with all the underlying circuit-patterns of the stack.
While it is possible to employ contact printing for imaging a circuit-pattern on the surface of the wafer, it is more practical to employ projection optical system for this purpose. One type of such projection optical system, now known as a Half-Field Dyson projection optical system, is disclosed in U.S. Pat. No. 4,964,705, entitled "Unit Magnification Optical System," which issued Oct. 23, 1990 to David A. Markle, and in its continuation-in-part U.S. Pat. No. 5,040,882, entitled "Unit Magnification Optical System with Improved Reflective Reticle," which issued Aug. 20, 1991 to David A. Markle (both of the aforesaid patents being assigned to the same assignee as the present patent application). The teachings of both of these patents are incorporated herein by reference. Specifically, an advantage of Half-Field Dyson projection optical system is that it is particularly suitable for projecting an image of a reflective reticle integrated-circuit layer pattern, that occupies a relatively large optical field, on the surface of a wafer.
The features of both a reticle integrated-circuit layer pattern and its image on a wafer surface have microscopic dimensions. This means that a high numerical aperture is required of any projection optical system, such as the Half-Field Dyson projection optical system, in order to obtain a high-resolution image of the integrated-circuit layer pattern on the wafer surface. Such a high numerical aperture projection optical system has a microscopic depth-of-focus. Further, the thickness of a wafer varies from one wafer to another. The problem is then to make sure that the surface of a wafer is controllably moved by the stepper to that position where the wafer's surface substantially coincides with the image plane of the projection optical system of the stepper. In the case of each of the stacked layer patterns above the first-layer pattern, the solution to this problem is relatively straight forward. Fiducial marks on an already-processed preceding layer pattern situated on the wafer's surface may be used in combination with a variety of different techniques to move the wafer's surface to that position at which these fiducial marks are in focus. However, this solution cannot be used to focus an unpatterned mirror-like wafer surface The present invention is directed to a focusing system that is capable of focusing such a first-layer pattern on a completely unpatterned mirror-like wafer surface, as well as focusing each subsequent-layer pattern on the wafer surface.